1. Field of the Invention
The present invention relates to a timing pulse generator and, more particularly, relates to a timing pulse generator which is suitable for controlling a non-impact printer.
Generally, a non-impact printer, such as a laser printer, comprises: a light source; a light modulator which receives a light beam from the light source and which produces a modulated light beam in accordance with both a character signal provided from a character generator and print timing control pulses; a light modulation controller which is comprised of a print timing control pulse generator and a character generator which generates a character signal which is synchronous with print timing control pulses, and; a light beam reflector made of a rotating polygonal mirror or galvanometer which alternately reflects the modulated light beam for scanning an area of a rotating photoconductor drum. A printing sheet is coupled with the photoconductor drum and an original character pattern is reproduced on the printing sheet by utilizing a well-known electrophotographic process.
However, the above described non-impact printer has an inherent problem in that the scanning light beam cannot scan the surface of the photoconductor drum at a constant scanning speed along each scanning line from one end to the other end. Specifically, the middle area of each scanning line is scanned at a relatively low scanning speed, while both side areas thereof are scanned at a relatively high scanning speed. This is because the scanning light beam is reflected from a point located on the polygonal mirror or galvanometer and is swung from one end to the other end of each scanning line (the details of this fact will be mentioned hereinafter). It should be noted that, when the character pattern is generated with constant scanning frequency, each scanning line should be scanned by the scanning light beam at a constant scanning speed and when the character pattern is printed with variable scanning speed, the frequency for generating the character pattern should be changed in proportional to the variable scanning speed, in order to reproduce the original character pattern without producing any distortion on the printing sheet.
2. Description of the Prior Art
In the prior art, for the purpose of solving the above mentioned problem, three typical methods have been proposed. According to a first method, a f-.theta. lens or a parabolic reflector is introduced in the light beam path between the polygonal mirror and the photoconductor drum, so that the scanning light beam can scan the scanning line on the drum, from one end to the other end, at a constant scanning speed.
In second and third methods, the scanning speed is variable and the character pattern is generated based on clock pulses having the frequency proportional to the variable scanning speed.
According to the second method, an analogue control device is employed in the printer. The analogue control device is mainly comprised of both a function generator and a voltage controlled oscillator (VCO). The output from the function generator is applied to the control input of the VCO. The VCO produces basic clock pulses from which the print timing control pulses are to be derived. The frequency of the basic clock pulses is variable so as to keep the ratio between the scanning speed and the frequency of the basic clock pulses constant. The variation of the frequency of the basic clock pulses is determined by the function generator.
According to the third method, a digital control device is employed in the printer. The digital control device is mainly comprised of both a crystal oscillator and a variable frequency divider. The crystal oscillator produces basic clock pulses having a very high frequency and stability. The variable frequency divider produces the print timing control pulses having a lower frequency than that of the basic clock pulses, however, the frequency of the print timing control pulses can be varied so as to keep the ratio between the scanning speed and the frequency of the basic clock pulses constant. The variation of the frequency of the printing control pulse can be defined by the dividing ratio of the variable frequency divider. The dividing ratio may be determined by a suitable memory device, such as a ROM (read only memory).
As is known, the character pattern is built up by many dots, that is a picture dot matrix, on the photoconductor drum. These dots are formed in each scanning line so that they are synchronous with the print timing control pulses in accordance with the character pattern signal. In a case where the character has a relatively simple pattern, such as an English letter or an Arabic numeral, each of these characters may be built up by a relatively small number of dots. Therefore, the print timing control pulses having a relatively low frequency may be used as the output from the print timing control pulse generator. However, in a case where the character has a relatively complicated pattern, such as a Chinese or Japanese Kanji character, each of these characters must be made up of a considerably large number of the dots. Therefore, the print timing control pulses having a considerably high frequency must be used as an output from the print timing control pulse generator. The print timing control pulses should be, for example, 40 MHz when the printing speed is set to be 10,000 lines/minute. In this case, according to the third method, the frequency of the basic clock pulse should be, for example, 320 (40.times.8) MHz, if the distortion of the character is to be limited to a width smaller than a 1/8 dot pitch. Thus, print timing control pulses having considerably high frequencies must be used in the printer so as to reproduce Chinese or Japanese Kanji characters without producing any distortion anywhere on each scanning line.
However, the above mentioned second method has a first defect in that it is very difficult to operate the function generator and the VCO with a high degree of accuracy on the order of 10.sup.4, without employing a high priced function generator and a high priced VCO. As a result, it is very difficult for the function generator and the VCO to correctly locate an n-th (n is a positive integer) dot of one scanning line at a position which aligns with the corresponding n-th dot of the preceding scanning line arranged adjacent to the scanning line without employing a high priced function generator and a high priced VCO. In addition, this method has a second defect in that it is necessary to perform maintenance adjustments of the function generator periodically. Finally, this method has a third defect in that, if the function of the function generator must be modified, another new function generator must be employed, instead of the old function generator.
The above mentioned third method has a first advantage in that it is very easy to correctly locate n-th dot of one scanning line at a position which aligns with the corresponding n-th dot of a preceding scanning line, merely by increasing the number of bits comprising the control data to be stored in the aforementioned ROM. This is because, since the frequency of the basic clock pulses is fixed, a highly stabilized oscillator, such as a crystal oscillator, can be used. A second advantage of this third method is that it is not necessary to carry out periodic maintenance, because the control data stored in the ROM cannot be changed by external conditions. A third advantage of this third method is that it is very easy to modify the function, merely by replacing the ROM with another ROM. However, this third method has a defect in that it is necessary to employ very high speed logic devices which can operate with the frequency of, for example, 320 MHz. However, from the technical and economical view points, it is preferable not to employ in the printer such very high speed logic devices.
The above mentioned first method has an advantage in that it is not necessary to use basic clock pulses having such a high frequency as 320 MHz (40.times.8). This is because the scanning light beam can always scan the photoconductor drum at a constant scanning speed due to the presence of the f-.theta. lens or parabolic reflector. However, this method has a defect in that the printer is high in cost and large in size. This is because a light control system including such a f-.theta. lens or parabolic reflector must be included in the printer.